1. Field of the Invention
The invention relates to the field of protective relaying.
In particular, it relates to a method for detecting ground faults on the conductors of an electrical machine
2. Discussion of Background
Such a method is described, for example, in the equipment description "100% stator and rotor ground-fault protection for large GIX 104 generators" ABB Relays AG, Baden. A related method is described in the equipment description "100% generator/stator ground-fault protection GIX 103" from the same company.
Ground faults occur in an electrical machine, in particular ;in generators of relatively high output, chiefly due to mechanical damage to the insulation between the conductors and iron parts. Such ground faults cause the flow of a fault current whose magnitude assumes different values depending on the type of fault. Given a known voltage, a fault resistance can be determined therefrom using Ohm's law, the magnitude of which can be used to assess the fault. In the normal case, the fault resistance has values in the region of M.OMEGA., while in the event of faults it can drop to values which are smaller by orders of magnitude.
Since the star point in the conductors, for example of the stator of a generator, connected in star is de-energized by definition, a fault can be detected in the vicinity of the star point only if the conductors are biased with respect to ground potential. This is performed in the case of two known protective devices and methods by means of a low-frequency voltage which is fed into the conductors.
A small current flows to ground during operation through the ground reactances of the conductors. In the event of a ground fault, these reactances are short circuited, and the current becomes larger than during operation in the absence of ground faults. In the GIX 103, use is made of an injection signal whose phase position remains unchanged. It is disadvantageous in this method, however, that a current circulating via the capacitors of the rotor circuits can occur which has the same frequency as that of the injection voltage. This can give rise to erroneous indications.
An attempt is made to avoid this in the GIX 104 by switching the phase of the injection voltage back and forth continuously between two values. It is possible in this way precisely to distinguish a fault current from other currents.
However, it is a common feature of both protective devices that the fault resistance can be determined subsequently only with the aid of empirical values or with the aid of recorded characteristics which are different for each machine. In addition, the measurement of the fault current is influenced by ageing, temperature, fluctuations in the injection voltage level, etc. The two systems additionally have the known shortcomings which are inherent to analog systems.